Grinding is a key process in mineral ore dressing and in particle processing in general, and is often carried out in a grinding mill in the presence of a grinding medium.
An important consideration in any grinding process is the amount of energy that is required to grind the material being ground to a particular fineness of grind. This is important because efficiency gains translate directly into cost and environmental savings.
A number of different factors can at any one time affect the amount of energy that is required to grind a particulate material to any given particle size. These factors include the properties of the mineral (hardness, fracture habit, etc.) type of mill (e.g., tumbling, vibratory, stirred, etc.), the grinding process conditions (e.g., dry or wet), the form of the grinding medium (e.g., the composition and physical shape of the particles comprised in the medium) and the form of the material being ground (e.g., slurry/material mix). It is not readily predictable how the modification of any one of these factors will affect the efficiency of the overall grinding process.
Aspects of the known grinding processes are discussed in the patent and academic literature, and in this respect, a number of studies have focussed on the form the grinding media should take.
Lameck et al in ‘Effects of grinding media shapes on load behaviour and mill power in a dry ball mill’, Minerals Engineering 19 (2006) 1357-1361, investigated the effects of grinding media shape (cylpebs, spherical and worn balls) on load behaviour and mill power draw at various mill speeds and load filling. The authors concluded that spheres require higher energy input than cylpebs, but that this effect is probably only relevant in tumbling mills, where interlocking packing is a barrier to energy transfer.
In Shi, F., ‘Comparison of grinding media-Cylpebs versus balls’, Minerals Engineering 17 (2004) 1259-1268, laboratory tests were conducted using a standard Bond ball mill to compare the milling performance of cylpebs (a slightly tapered cylindrical media with a length to diameter ratio of unity) against balls. It was found that cylpebs produce a similar product at the fine end compared with balls at identical charge mass and at the identical specific energy input level.
U.S. Pat. No. 4,695,294 discloses a grinding mixture comprising silicon carbide pellets having a maximum dimension of from 5 to 50 mm and a suspension of silicon carbide powder which is suitable for use in a vibratory mill. The silicon carbide pellets may have a cylindrical shape and the diameter of the cylinder may be from 0.3 to 3 times the length of the cylinder. The grinding media is described as having good resistance to degradation during grinding of silicon carbide powders by vibration, and can be used to grind silicon carbide without contamination.
In a similar manner, U.S. Pat. No. 7,267,292 describes a grinding media including shaped media such as spheres or rods ranging in size from about 0.5 um to 100 mm in diameter, which are formed from a multi-carbide material consisting essentially of two or more carbide-forming elements and carbon. The media are said to have extremely high mass density, extreme hardness and extreme mechanical toughness.
WO-A-2001/085345 describes a grinding media in the form of non-spherical shapes such as cylindrical and toroidal shapes, and combinations of grinding media with different shapes and sizes.
EP-A-1406728 describes a process for the preparation of a drug carrier composite by grinding a drug-carrier mixture in a vibratory mill in the presence of a grinding media of cylindrical shape having a dimensional ratio (diameter to height) of between 0.5 and 2. The process is said to lead to a drug having a high and constant degree of activation.
From a cost and environmental perspective, there is an ongoing need for the development of grinding processes which require less energy to grind materials to any given particle size.
As discussed in more detail below, the present inventors have surprisingly found that the amount of energy required to grind a particulate material in a stirred mill to a pre-determined particle size distribution (for example as defined by the d50) can be reduced by using a grinding media comprising rod-shaped particles having an aspect ratio of equal to or greater than 2:1.